Recently, as a rewritable nonvolatile memory, the magnetic random access memory (hereinafter called MRAM) including magnetoresistance effect elements arranged in a matrix is noted. The MRAM memorizes information by using combinations of magnetization directions of two magnetic layers and read memorized information by detecting resistance changes (i.e., current changes or voltage changes) between when magnetization directions between the magnetic layers are parallel and when magnetization directions between the magnetic layers are antiparallel are detected.
As one of the magnetoresistance effect elements forming the MRAM, the magnetic tunnel junction (MTJ) element is known. The MTJ element includes two ferromagnetic layers stacked with a tunnel insulating film therebetween and uses the phenomenon that the tunnel current flowing between the magnetic layers via the tunnel insulating film changes based on relationships between the magnetization directions of the two ferromagnetic layers. That is, the MTJ element has a low element resistance when the magnetization directions of the two ferromagnetic layers are parallel and a high element resistance when the magnetization directions of the two ferromagnetic layers are antiparallel. These two states are related to data “0” and data “1”, whereby the MTJ element can be used as a memory element.
Writing information into the MTJ element is made by applying a magnetic field to the MTJ element to inverse a magnetization direction of one ferromagnetic layer (free magnetization layer). Specifically, currents are flowed respectively in two lines arranged crossing each other, and a synthetic magnetic field formed by these currents is applied to the MTJ element. The direction of the current flowed in one line is made opposite, whereby a direction of the synthetic magnetic field to be applied to the MTJ element is also inversed. Thus, the magnetization direction of the free magnetization layer of the MTJ element can be arbitrarily controlled.
As conventional magnetic memory devices using the MTJ element are proposed 1T-1MTJ magnetic memory device comprising one memory cell including one MOS transistor and one MTJ element, 2T-2MTJ magnetic memory device comprising one memory cell including two MOS transistors and two MTJ elements, and others.
Related arts are disclosed in, e.g., Japanese published unexamined patent application No. 2001-236781, Japanese published unexamined patent application No. 2001-273758; Japanese published unexamined patent application No. 2003-197876; Japanese published unexamined patent application No. 2004-030822; M. Aoki et al., “A novel voltage sensing 1T/2MTJ cell with resistance ratio for high stable and scalable MRAM”, 2005 Symposium on VLSI Circuits Digest of Technical Papers, pp. 170-171; Roy Scheuerlein et al., “A 10 ns read and write non-volatile memory array using a magnetic tunnel junction and FET switch in each Cell”, ISSCC Dig. Tech. Papers, pp. 128-129, 2000; M. Durlam et al., “A low power 1 Mbit MRAM based on 1T1MTJ bit cell integrated with copper interconnects”, Symposium on VLSI Circuits Dig. Tech. Papers, pp. 158-161, 2002; and N. Tanabe et al., “A high density 1T/2C cell with Vcc/2 reference level for high stable FeRAMs”, IEDM Tech. Dig., pp. 863-866, 1997.
However, the 2T-2MTJ magnetic memory device includes a large number of the elements forming one memory cell, which makes it difficult to improve the integration. The 1T-1MTJ magnetic memory device, however, is easy to be integrated in comparison with the 2T-2MTJ magnetic memory device. However, reference signals are generated by reference cells, and fluctuation of the reference cells directly influences the read margin. Especially, one reference cell is provided for a plurality of bit lines, some of the bit lines are near the reference cell and others are remote from the reference cell, which largely influences the fluctuation of the characteristics of the MTJ elements. Thus, the reading by the differential amplification using a pair of adjacent bit lines, which is characterized by being strong to noises, cannot be made, and there is a risk that the resistance to noises would be decreased.